Abstract: The present invention relates to a method for selecting a suitable expression construct from a plurality of expression constructs for optimizing the production of a protein or a fragment thereof in a host cell, a method for the production of proteins or fragment thereof using the selected expression vector, to novel human embryonic kidney cells that are deficient in N-acetyl-glucosaminyltransferase I and stably transfected with EBNA (HEK 293E GnTI?cells) that are well suitable for use in the said method, in particular for the production of proteins or protein fragments that are suitable for X-ray studies. The invention also relates to a method to produce HEK 293E GnTI? cells and a method to confer to HEK293E GnTI? cells, the capacity to grow in suspension and to a method to confer to HEK293E GnTI? cells, the capacity to grow in serum free medium. The invention also relates to a kit comprising different vectors suitable for use of the above method for the production of proteins or protein fragments.